Eur. J. Entomol. 112(3): 520–524, 2015 doi: 10.14411/eje.2015.058 ISSN 1210-5759 (print), 1802-8829 (online)

Suction sampling of grassland invertebrates using the G-vac: Quantifying and avoiding peripheral suction effects

Andrew Cherrill

Entomology Research Group, Crop and Environment Sciences, Harper Adams University, Edgmond, Shropshire TF10 8NB, UK; e-mail: [email protected]

Key words. , Auchenorrhyncha, vacuum sampling, peripheral suction effect, D-vac, G-vac, Vortis

Abstract. Suction sampling with modified garden leaf-blowers (G-vacs) is a widely used method for research on invertebrates in agricultural and grassland habitats. Approaches vary from sweeping the collecting nozzle across the surface of the vegetation within a known area, applying the nozzle firmly to the ground to delimit a sample area equal to nozzle crosssection, or applying the nozzle repeatedly within a larger open-ended cylinder placed on the ground. These approaches vary in potential for the inadvertent capture of specimens from outside of the sample area as a result of air being drawn into the nozzle from adjacent vegetation. This has never been studied adequately and is defined here for the first time as the Peripheral Suction Effect (PSE). Invertebrate species are likely to differ in susceptibility to PSE and so both sample size and composition could be impacted. This study compares two series of samples of Auchenorrhyncha taken using the same G-vac suction sampler (nozzle area, 0.01 m2) from areas enclosed and unenclosed by an open- ended cylindrical enclosure (area 0.17 m2) intended to prevent PSE. The unenclosed samples contained greater numbers of including Arthaldeus pascuellus, Javesella pellucida and immature . Numbers of immature Delphacinae did not differ between enclosed and unenclosed samples. The species composition of the two series of samples was similar, but the proportional representation of immature Delphacinae in unenclosed samples was diluted by the inflated numbers of other taxa that were drawn from the surrounding area. Peripheral suction effects are taxa-specific and therefore have the potential to bias measures of invertebrate com- munity composition. Use of an open-ended cylinder to delimit the sample area is recommended as a simple, inexpensive and effective method of avoiding problems associated with PSE when using a G-vac. The potential for PSE with other models of suction samplers is discussed and questions for further research are identified.

Introduction 2006, 2009). The sample is then taken by repeatedly ap- Suction samplers are widely used in entomological re- plying the nozzle within the cylinder until the whole area search in grassland and agricultural systems (Macleod et has been covered. Sampling from within a cylinder has the al., 1994; Samu et al., 1997; Standen, 2000). Key determi- potential advantage that the inadvertent capture of speci- nants of the numbers and species of invertebrates collected mens from outside of the sample area is prevented (Stewart are nozzle diameter, suction power, vegetation structure, & Wright, 1995). The term Peripheral Suction Effect (PSE) weather and the characteristics of the target organisms is proposed here for the first time to describe this problem. (Henderson & Whitaker, 1977; Hossain et al., 1999; Bell PSE is defined as the capture of specimens from outside et al., 2002; Sanders & Entling, 2011). Various types of of the sample area as a result of air (and material) being apparatus are available with modified garden leaf blower- drawn into the nozzle from adjacent surfaces. PSE can oc- vacuum apparatus, commonly known as a G-vacs, being cur because of an incomplete seal between the nozzle and used widely (Stewart, 2002). the surface being sampled and when the nozzle is raised Suction samples using G-vacs are routinely collected us- and lowered during sampling. ing one or other of three approaches (Stewart, 2002). The To date there has been no adequately designed study of simplest approach has been to sweep the nozzle across the PSE to determine unequivocally the existence and magni- surface of the vegetation within a defined area (e.g. El- tude of such an effect. As outlined above, sampling cyl- liot et al., 2006; Doxon et al., 2011). A second approach inders have been used to delimit sample areas in some, has been to apply the nozzle of the apparatus flat against but not all studies. Quantitative information is therefore the ground for a given time period. This area, equal to the required to determine the extent to which catch size and cross-sectional area of the nozzle, is then regarded as the composition may be influenced by the use or otherwise of basic sample unit, although often a series of such “touch- a sampling cylinder, thereby guiding best practice in future downs” are pooled to give a single sample of larger area studies. (e.g. Macleod et al., 1994; Samu & Sarospataki, 1995; Bell In the present study, a comparison is made between et al., 2000; Nickel & Hildebrandt, 2003). A third approach samples of Auchenorryncha taken using a G-vac with and has been to sample within an open ended cylinder placed without the use of a sampling cylinder to control PSE. The on the ground immediately before sampling (e.g. Stewart results show that both catch-size and species-composition & Wright, 1995; Hossain et al., 1999; Littlewood et al., are influenced by use of a sampling cylinder with the G- vac. The implications for sampling, and further research,

520 are discussed both for the use of G-vacs, but also other designed to be relatively conservative so as not to inflate the risk vacuum sampling apparatus where the risks of PSE are of PSE. likely to differ. At the end of each 30 s sample period, the nozzle was raised clear of the vegetation and the motor was allowed to idle while Material and Methods the net was emptied into a cotton bag. The net was turned inside out before being reattached within the nozzle of the suction sam- The study was carried out at Harper Adams University, Shrop- pler. Reversing the net ensured that cross-contamination of suc- shire, England, UK (Latitude 52°78´N, Longitude 2°43´W). The cessive samples was impossible. grassland was a sown representative of Lolium perenne-Cynosu- Samples were stored within a cool box in the field before be- rus cristatus, Typical sub-community, MG6a in the UK National ing frozen. After sorting Aunchenorrhycha were stored in 70% Vegetation Classification (Rodwell, 1992). Based on a series methanol and identified using Biedermann & Niedringhaus of ten 0.25 m2 quadrats, the mean covers of the most abundant (2009). Remaining invertebrate, plant and soil material (hence- species occurring in more than half of quadrats were L. perenne forth labelled debris) was refrozen. After all samples had been 55.8% (SD = 16.3), Cynosurus cristatus 23.0% (SD = 16.5) and sorted, debris was air dried for six weeks at room temperature and Agrostis stolonifera 7.2% (10.2). The grassland was established weighed to 0.001 g. five years previously and is mown with the first cut typically in Statistical analysis used Pearson’s correlation and Student’s t- mid-July. The pasture was selected because of its uniformity of test for independent samples. Variables that did not conform to a composition and height structure. Mean height, assessed using a normal distribution, and which could not be transformed to meet drop plate (Cherrill & Rushton, 1993), was 16.5 cm (SD = 3.75, this assumption, were analysed using an equivalent non-paramet- N = 30). The sward was sufficiently open that placement of the ric test. sampling cylinder and suction nozzle could be achieved without pressing the vegetation flat against the ground. Results The suction sampler was a modified McCulloch GBV 325 gar- den blower/vacuum commonly referred to as a G-vac (Stewart In total, 1543 leafhoppers were collected. Nine species & Wright, 1995, Stewart, 2002). The end of the collecting pipe were identified and a large number of immature specimens was sawn off perpendicular to its length and had a cross sectional were classified to sub-family (Table 1). area of 0.011 m2. All samples were taken from a standard sample Mean numbers of all leafhoppers, and numbers of area of 0.174 m2 defined by the internal area of a rigid plastic Arthaldeus pascuellus, Javesella pellucida and immature drain pipe (diameter 47 cm). A section 60 cm in length and ap- Deltocephalinae, were greater in unenclosed samples than proximately 5 kg in weight was used as the sampling cylinder. enclosed samples (Table 1). The number of immature Del- To ensure an equal area was used for unenclosed samples, a 3 cm ring was cut from the same pipe. phacinae, number of species and weight of debris did not Samples were taken between 13:00 and 15:00 on 9th July 2013. differ between the two sets of samples. Species composi- Air temperature 1 m above ground at 13:00 was 27°C and fluctu- tion was also broadly comparable. However, the propor- ated by only 2°C. The vegetation, leaf litter and soil surface were tional representation of immature Delphacinae in unen- dry. There was no wind. Cloud cover was zero. closed samples was diluted by the presence of other taxa Samples were taken at intervals of 2 m along two parallel 80 m drawn in from outside of the sampling area. In enclosed transects separated by 5 m. Samples that were enclosed and unen- samples, immature Delphacinae made up a higher mean closed by the cylinder were taken alternately along each transect proportion of all specimens (mean = 46.4%, SD = 18.7), by the same operator. Twenty enclosed and 20 unenclosed sam- compared with in unenclosed samples (36.1%, SD = 14.1) ples were taken (10 of each per transect). Each sample was taken by placing the cylinder or ring on the ground and then immediate- (t-test using arcsin transformed data, P = 0.05). As a per- ly applying the suction sampler for 30 s. The motor was run at full centage of the total numbers captured immature Delphaci- throttle from when the nozzle was first lowered on to the ground. nae were 52.1% (N = 612) and 37.2% (N = 922) of those The nozzle was held in place for 1 to 2 s before being raised from enclosed and unenclosed samples respectively. rapidly to approximately two-thirds the height of the grass sward There was no correlation between the weight of debris (i.e. approximately 10–15 cm above ground level) and then low- and numbers of leafhoppers either overall (r = 0.12, P = ered immediately to an adjacent and partially overlapping posi- 0.47) or for enclosed (r = 0.40, P = 0.08) or unenclosed tion within the sample area. The motor was run at full throttle as it samples (r = –0.04, P = 0.88) considered separately. was raised from each touch-down to prevent escape of specimens from the net as is standard practice with G-vac samplers (Stewart Discussion & Wright, 1995). After 30 s of this repeated “touch-down” sam- pling the whole 0.17 m2 sample area was covered. Stewart (2002) reviewed vacuum sampling methods The “touch-down” duration was based on studies showing that for grassland Auchenorrhyncha and highlighted the risk over 90% of specimens are usually captured within the first sec- of specimens being sucked up from adjacent vegeta- ond (Bell et al., 2002; Brook et al., 2008). Other studies have tion. However, the extent of this effect, defined here for frequently used periods of 5 s or 10 s (e.g. Nickel & Hildebrandt, the first time as the Peripheral Suction Effect (PSE), has 2003; Hollier et al., 2005). The total sampling duration of 30 s not been studied in detail. I am aware of only two studies was also based on previous studies. For example Stewart & that relate directly to the issue. In the first study, Duffey Wright (1995) used a sample period 20 s with an enclosed area (1974) compared grassland invertebrates from two series of 0.1 m2, while Hossain et al. (1999) used periods of both 90 s and 120 s within enclosed areas of 0.2 m2. Longer touch-downs of samples of equal area made using a Dietrick vacumm and total sample periods may have increased marginally the pro- sampler (D-vac) with a collecting nozzle of approximately 2 portion of invertebrates collected from within the nozzle area, 0.09 m . One series comprised the catch from nine quad- whilst increasing the probability of PSE. Sampling periods were rats, each 1 m2 and lacking a side-wall, from within which

521 Table 1. Mean weight of debris and average number of individuals and species obtained by suction sampling within enclosed and unenclosed areas (n = 20 in each case). Enclosed Unenclosed Comparison between Mean SD Mean SD groups (P) Adult specimens Philaenus spumarius 0.05 0.22 0.05 0.22 – Neophilaenus lineatus 0.05 0.22 0.0 0.0 – Javesella pellucida 0.80 1.20 2.90 1.92 < 0.001 Arthaldeus pascuellus 6.45 4.41 11.45 7.46 0.014 Streptanus sordidus 0.35 0.81 0.60 1.10 – Euscelis incisus 0.15 0.49 0.25 0.55 – Cicadula persimilis 0.05 0.22 0.0 0.0 – Zyginidia scutellaris 0.10 0.31 0.0 0.0 – Immature specimens Delphacinae* 15.95 13.88 17.30 10.77 0.48 Deltocephalinae* 6.65 4.55 14.00 7.11 < 0.001 Total number of individuals 30.60 17.67 46.55 18.68 0.009 Number of species 1.95 0.76 2.35 0.75 0.10 Weight of debris and by-catch (g) 3.55 1.36 3.16 0.96 0.31 Comparison between groups used Student’s t-test except for J. pellucida for which Mann-Whitney U-test was used because data could not be transformed to meet assumptions. No analysis was performed where the total number of individuals was less than 20; * indicates that data was square-root transformed prior to analysis. contiguous touch-down samples were taken. The second the investigation of PSE. Enclosed and unenclosed sam- series comprised nine samples of 1 m2 each obtained by ples were identical in area and perimeter and differed only pooling the catch from eleven widely-spaced touch-downs. in the presence/absence of the cylinder. The cylinder was This second series contained significantly greater numbers designed to prevent capture of invertebrates from outside of Coleoptera, Diptera, Hymenoptera and Homoptera. In of the stated sample area. The results show an effect on the second study, Samu et al. (1997) investigated the den- numbers of Auchenorrhyncha captured and support the sity of spiders within an alfalfa crop. They used a G-vac view that suction sampling can inadvertently draw in num- with a nozzle area of 0.01 m2 and compared a series of bers from outside of the sample area to inflate the sample 48 standard “touch-down” samples with those taken by size. Moreover, the effect of PSE was not consistent for all applying the same nozzle repeatedly within a 0.5 m high taxa. Numbers of immature Delphacinae showed no dif- metal cylinder with an area of 0.48 m2. The large cylinder ference between sample types, but immature Delphacinae and series of smaller samples had the same total area, yet represented a greater proportion of all specimens in en- more spiders were found in the latter. The results from both closed compared with unenclosed samples. Their numbers studies were interpreted as being caused by additional ani- were diluted by other taxa drawn from beyond the sample mals being drawn from adjacent vegetation into the smaller area within unenclosed samples. Measures of invertebrate sub-samples which had much larger total perimeter-to-area community composition, such as proportional diversity in- ratios. Whilst this conclusion is intuitive, a scientifically dices, may therefore also be biased by PSE. rigorous investigation would involve a comparison of sam- The results are particularly pertinent for use of G-vacs pling strategies that are identical in terms of the perimeter- and D-vacs because many authors have pooled series of to-area ratio, but different in terms of the presence or ab- sub-samples to increase sample area and catch (Nickel & sence of an experimental manipulation to prevent capture Hildebrandt, 2003; Hines et al., 2005). Moreover, not all of specimens from adjacent vegetation. Samu et al. (1997), studies using a G-vac utilise a sampling cylinder (Macleod used a sampling cylinder in one series of samples, but the et al., 1994; Samu & Sarospataki, 1995; Bell et al., 2002) use of the cylinder was confounded with a seven-fold dif- and this would be the exception for application of a D-vac ference in total perimeter length between treatments. As (e.g. Morris et al., 2005). Nickel & Hildebrandt (2003), for shown by Duffey (1974) more spiders might have been example, defined a G-vac sample as the pooled catch from anticipated from the series of smaller samples even if the ten widely-spaced touch-downs of 0.015 m2 each defined cylinder had not been used. Samu et al. (1997) has been by the nozzle of their machine. These ten touch-downs had cited widely as evidence for the capture of specimens from a combined perimeter of 4.3 m, while a single circle with adjacent vegetation (defined here as PSE) (Stewart, 2002), the same total area has a perimeter of only 1.4 m. The po- but the sampling design was not appropriate to allow an as- tential for PSE may therefore be greater than is apparent sessment of its importance due to the confounding effects from a consideration of total sample area alone. of variation in perimeter-to-area ratios. On balance PSE is likely to be less of a concern with a The present study is the first to circumvent the problem D-vac compared to a G-vac because the former has a larger of confounding sample perimeter-to-area ratio effects in nozzle with lower perimeter-to-area ratio. The suction pow-

522 er of a D-vac is also considerably lower than most modern Acknowledgements. I am grateful to A. Broadhurst and G-vacs (Stewart, 2002). It is therefore predicted that PSE colleagues for fabrication of prototype sampling cylinders and to will be smaller for a D-vac than for a G-vac, although this S. Leather, T. Pope and R. Graham for stimulating discussions. awaits experimental verification. The relevance of the pre- Feedback from anonymous referees also greatly improved this paper. sent study to use of Vortis suction samplers (manufactured by Burkard, UK) also warrants consideration. Sampling References using the Vortis typically involves pooling a series of sub- Barham D.F. & Stewart A.J.A. 2005: Differential indirect effects samples (e.g. Borges & Brown, 2003; Barham & Stewart, of excluding livestock and rabbits from chalk heath on the as- 2005; Maczey et al., 2005), however the potential for PSE sociated leafhopper (Hemiptera: Auchenorrhyncha) fauna. — is reduced because the motor can be idled whilst the noz- J. Conserv. 9: 351–361. zle is being lowered and raised between points (Stewart, Bell J.R., Wheater C.P., Henderson R. & Cullen R. 2002: Test- 2002). Repeating the present study to quantify the extent ing the efficiency of suction samplers (G-vacs) on spiders: the of PSE with the D-vac and Vortis is feasible, but for the effect of increasing nozzle size and suction time. In Toft S. & latter would require the use of a shorter sampling cylinder. Scharff N. (eds): European Arachnology 2000. Aarhus Univer- This is because the Vortis has a wide expansion chamber sity Press, Aarhus, pp. 285–290. above the collecting nozzle which would prevent the noz- Biedermann R. & Niedringhaus R. 2009: The Plant- and Leaf- Hoppers of Germany – Identification Key to All Species. Wis- zle being placed flat on the ground against the inside edge senschaftlich Akademischer Buchvertrieb-Fründ, Scheeßel, of a tall sampling cylinder. Nonetheless, it is predicted that 409 pp. PSE with the Vortis would be lower than for the G-vac in Borges P.A.V. & Brown V.K. 2003: Estimating species richness the present study. of in Azorean pastures: the adequacy of suction Further research might also explore the reason for the sampling and pitfall trapping. — Graellsia 59: 7–24. taxon-specific PSE shown with the G-vac. 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It would therefore and pitfall trap sampling of moorland Hemiptera: evidence for be helpful to expand this investigation to include a broader vertical stratification within vegetation. — The Entomologist range of taxa, vegetation types and sample durations. Sub- 113: 70–81. tle variations in the application of vacuum samplers may Doxon E.D., Davis C.A. & Fuhlendorf S.D. 2011: Comparison also have unforeseen consequences. For example, varia- of two methods for sampling invertebrates: vacuum and sweep- tion in the pressure used to apply the G-vac nozzle to the netting. — J. Field Ornithol. 82: 60–67. Duffey E. 1974: Comparative sampling methods for grassland ground, and in speed with which the nozzle is raised and spiders. – Bull. Brit. Arachnol. Soc. 3: 34–37. lowered between samples, may influence the risk of PSE. 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